Directional optical couplers are key elements in planar lightguide circuits (PLCs). For example, directional couplers with coupling ratios which vary from zero to one are required for Fourier filters and ring filters, and directional couplers having a 50% power splitting ratio are needed in Mach-Zehnder Interferometers (MZI). Couplers are widely used in optical transmission systems.
In many of these applications, the coupling ratios in the coupler as fabricated must be very close to the design values so that the filter has the desired spectral response. An important evolution for silica based PLCs is for more compact devices, which require a higher refractive index difference (.increment.) between the core and cladding. A disadvantage of using high .increment. is that the fabrication variations on the coupling ratio increase. Typically, fabrication tolerances cause variations in the coupling ratios of .+-.5% for 3 dB couplers in silica with low .increment.s (.ltoreq.0.7%). A method for actively tuning the coupling ratios after fabrication would be beneficial by: 1) increasing device yield, and 2) providing additional functionality by allowing the coupling ratio to be chosen from a large range of coupling values after fabrication of the coupler. Silica waveguides offer low loss, but the only post-fabrication technique to actively tune the PLC is to use thermal heaters and rely on the thermal dependence of the refractive index (dn/dT). Heaters can be deposited on the surface of the waveguide and used to tune the coupling ratio. This technique can be used also to tune the phase in a MZI. See e.g. B. Zheng and C. Zheng, "A study of directional coupler modulator with thermo-controlled coupling coefficient, Integrated Optoelectronics, Beijing, China: SPIE Vol. 2891, 1996, pp. 178-182. However, the refractive index of silica is relatively insensitive to temperature, thus requiring a substantial temperature change to effect the desired adjustment. This requires both higher power and longer heating times. A tunable coupler with more sensitive and more rapid thermal response would represent a significant advance in the art.